The present invention relates to a method of displaying a diffraction pattern that can be derived from a microscopic area on a specimen, by an electron microscope.
Sometimes, electron microscopy employs large-angle convergent-beam electron diffraction to analyze a crystalline specimen. That is, such a specimen is scanned with an electron beam which falls on the specimen at varying incidence angle. The electron beam transmitted through the specimen without scattering is detected, and the resultant signal is supplied to a display means synchronized with said scan to display a diffraction pattern. In the conventional large-angle convergent-beam electron diffraction, a portion of an electron micrograph formed by an objective lens is allowed to pass through a diaphragm. Then, a portion of the diffraction pattern formed by the passed electron beam is detected to thereby produce a signal. The diffraction pattern obtained by this method shows a change in the intensity of an electron beam which has passed through a specimen without being scattered when the incidence angle at which the beam falls on the specimen is varied. The symmetry, point and space groups of a crystal can be effectively determined by analyzing this diffraction pattern. FIG. 1 shows one example of electron microscope for displaying such a diffraction pattern. This microscope includes a convergent (condenser) lens diaphragm (aperture plate) 1 and two stages of deflection coils 2 and 3. A specimen 4 is placed within an objective lens. Lenses (principal planes of the magnetic lens fields) produced before and behind the objective lens are indicated by reference numerals 5 and 6, respectively. The electron beam passed through the aperture formed in the diaphragm 1 is deflected by the deflection coils 2 and 3 and made parallel to the optical axis Z. Then it enters the lens 5 in front of the objective lens and is substantially collimated by the lens 5. Thereafter, it is further deflected and falls on a specific region on the specimen 4. Under this condition, the position at which the electron beam enters the front lens is varied, i.e., the beam is scanned across the front lens. As a result, the incidence angle of the beam upon the specimen 4 is varied, while the incidence position remains fixed. Disposed or produced at the rear of the rear lens 6 are a field of view-limiting diaphragm 7, a projector lens magnetic field 8, projector lens alignment coils 9, and a fluorescent screen 10.
In the structure described above, those portions of the electron beam falling on the specimen 4 which penetrated the specimen or were diffracted by the specimen are brought to focus by the magnetic lens 6 that is set up at the rear of the objective lens. Thus, an image of the specimen 4 is formed on the field of view-limiting diaphragm 7 that is disposed behind the lens 6. The diaphragm 7 acts to allow only a portion of the image of the specimen to pass through it and travel along the optical axis. Then, the projector lens field 8 projects the beam onto the fluorescent screen 10, so that a diffraction pattern is formed on it. This pattern is moved, since the incidence angle of the beam upon the specimen 4 is being changed. The projector lens alignment coils 9 are so adjusted that the diffraction pattern ceases to move, in spite of the changing incidence angle. The coils 9 receive signals that are synchronized with scanning signals applied to the deflection coils 2 and 3. The magnitude of the signals applied to the coils 9, the ratio of the magnitude of the Y scanning signal to the magnitude of the X scanning signal, and other factors are adjusted to attain the stoppage of the movement of the diffraction pattern. Subsequently, only the electron beam transmitted through the specimen 4, for example, is permitted to pass through the aperture 11 formed in the fluorescent screen 10, and then it is detected by a detector 12. The resultant signal is supplied as a luminance-modulating signal to a cathode-ray tube that is synchronized with the scanning signals applied to the coils 2 and 3, whereby a large-angle convergent-beam electron diffraction pattern is formed on the cathode-ray tube. This method of display is disclosed in "Journal of Electron Microscopy" Vol. 29, No. 4, 408-412, 1980.
The diffraction pattern obtained by the above-described method is derived from the beam that comes from a specimen region restricted by the diaphragm 7. Since the magnification of the specimen image focused on the diaphragm 7 is of the order of several tens, the restricted region is relatively broad. Even if the diameter of the aperture in the diaphragm 7 is as small as microns, the obtained diffraction pattern corresponds to a relatively broad area having a diameter of 0.5 .mu.m or more.